A man is not a Jew because he is one outwardly, nor is circumcision only outward and physical.
No, a man is a Jew because he is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the written code. Such a man’s praise does not come from men, but from God. (Romans 2:28-29)
Therefore be nice to everyone, you might be kind to a jew unaware.
I'm always kind to Jewish people my ethnicity. You maybe not so much. According to
halakhah (Jewish religious law), a Jew is anyone born of a Jewish mother or who has converted to Judaism. Meanwhile, it is commonly accepted that the Bible traces Jewishness genetically through the father.
According to this theory, Jewishness came to be traced through the mother at a time of historical turmoil when Jewish women were being sold as concubines to Gentiles, so that there could be doubt whether a father was Jewish. However, one can make a case that even in the Bible Jewishness depends on the mother. Some in the Reform movement have cut the Gordian knot by accepting as a Jew anyone from either parent. "Jew" (Hebrew, Y'hudi), literally means "one who praises"
You need to read the previous verses to grasp what Sha'ul (Paul) is trying to convey
to Messianic Jews.
Romans 2: 23-29
You who take such pride in Torah, do you, by disobeying the Torah, dishonor God? ---as it says in the Tanakh, "For it is because of you that God's name is blasphemed by the Gentiles." For circumcision is of value if you do what the Torah says, but if you are a transgressor of Torah, your circumcision has become uncircumcision! Therefore, if an uncircumcised man keeps the righteous requirements of the Torah, won't his uncircumcision be counted as circumcision?
Indeed, the man who is physically uncircumcised but obeys the Torah will stand as a judgment on you who have had a b'rit-milah and have Torah written out but violate it! For the real Jew is not merely Jewish outwardly: true circumcision is not external and physical. On the contrary, the real Jew is one inwardly; and true circumcision is of the heart, spiritual not literal; so that his praise come not from other people but from God.
I will rephrase my previous reply to you.
It will serve you well to be kind to ALL
Jews!